A multi-phase power converter includes a plurality of signal-phase switching regulators connected in parallel to a common output, each of the signal-phase switching regulators being regarded as a channel of the multi-phase power converter. For thermal balancing in a multi-phase power converter, the currents in each channel should be equal to each other and therefore, a current balance mechanism is necessary for implementing balanced currents between the channels. Generally, switching power converters employ pulse width modulation (PWM) control with constant or variable switching frequencies. FIG. 1 is a diagram showing the constant switching frequency PWM control, in which the signal PWM1 for driving a channel of a multi-phase power converter has a constant switching period Tsw, and by changing the on time Ton or the off time Toff of the signal PWM1, the duty ratio of the signal PWM1 is modulated to regulate the output voltage and the output current of the channel. FIG. 2 is a diagram showing the variable switching frequency PWM control, in which the signal PWM2 for driving a channel of a multi-phase power converter has a constant on time Ton, and by changing the switching period Tsw of the signal PWM2, the duty ratio of the signal PWM2 is modulated to regulate the output voltage and the output current of the channel.
FIG. 3 is a simplified circuit diagram of a conventional two-phase power converter 10 which includes channels 12 and 16. In the channel 12, a switching circuit 14 generates a channel current I1 according to a control signal S1, and in the channel 16, a switching circuit 18 generates a channel current 12 according to a control signal S2. In the two-phase power converter 10, the reason causing the channel currents I1 and I2 unbalanced includes mismatched impedances R1 and R2 of the channels 12 and 16 to each other. Assuming that the two-phase power converter 10 employs constant on time control and only an output voltage feedback loop, both the channels 12 and 16 will have a same switching period. In other words, both the channels 12 and 16 will have a same duty ratio. Without any current balance mechanism, the two-phase power converter 10 will enter a state having balanced voltages V1 and V2 and imbalance currents I1 and 12 due to the mismatched impedances R1 and R2.
Therefore, it is desired a current balance mechanism for multi-phase power converters with constant on-time control.